Rotary biological contactors, known as RBCs, have been in rather widespread use throughout the United States in various forms.
An extremely important consideration in such rotary biological contactors is the expected useful life of the rotary biological contactor. In most instances, the rotary biological contactor is intended to be designed for a minimum service life of twenty years and preferably will have an expected service life well in excess of that minimum.
A rotary biological contactor is used with a tank filled with waste water, and provision is made for flow of the waste water into and out of the tank. The rotary biological contactor is located in the tank and has a central shaft, and a plastic media carried by the shaft for rotation therewith, and a driving means for rotating the shaft and plastic media in the tank.
The rotary biological contactor is immersed in the waste water in the tank, and in most cases, the plastic media is submerged in the water and the shaft is out of the water so that the water level is slightly below the rotation axis of the rotary biological contactor. Usually, the rotary biological contactor is approximately 35 to 40% submerged.
In most instances in the past, the shaft of the rotary biological contactor is mounted in bearings which are affixed to the walls of the tank so that the rotary shaft is stationary with respect to the tank. This arrangement is illustrated in the Prosser U.S. Pat. No. 3,886,074, Thissen U.S. Pat. No. 4,115,268, Gass U.S. Pat. No. 3,827,559, and Iwatani U.S. Pat. No. 4,200,532; and is referred to but not illustrated in Holmberg U.S. Pat. No. 3,956,127 at column 3, lines 40-45.
In some instances, the shaft is not supported on the tank wall, but the rotary biological contactor is allowed to float freely in the waste water, with the flotation being supplied by the plastic media, as illustrated in the Holmberg Pat. No. 3,956,127, and in the Carter Biodrum commercial product, which is based upon the Holmberg patent. The Carter Biodrum is manufactured and sold by Ralph B. Carter Company, 192 Atlantic Street, Hackensack, N.J. 07602.
The rotary biological contactors which are known have utilized numerous different types of plastic media on the rotary shaft for rotating through the waste water and building up a bio-mass on the surfaces of the plastic media. In the Prosser Pat. No. 3,886,074, the plastic media is in the form of substantially flat wafers or discs affixed to the shaft; in Gass Pat. No. 3,827,559, the plastic media consists of numerous plastic discs on the square shaft; in Thissen Pat. No. 4,115,628, the plastic media is a long plastic strip spirally wound in spaced convolutions welded together, and in a multiplicity of modules along the length of the shaft; in Holmberg Pat. No. 3,956,127, the plastic media consists of a large number of hollow plastic balls, like ping pong balls, confined in a cage secured to the shaft; and in Iwatani Pat. No. 4,200,532, the plastic media includes hollow plastic balls, which are apertured or pierced, so that the waste water may be both inside and outside of these walls. In all such plastic media, the wafers or discs convolutes or balls a multiplicity of passages are provided to alternately receive and discharge air and water.
The drive for revolving the rotary biological contactors is provided in numerous forms, such as in Iwatani Pat. No. 4,200,532, the shaft extends beyond the tank wall and is driven from a motor by a chain and sprocket drive transmission. In Prosser Pat. No. 3,886,074, the rotary biological contactor is rotated by bubbling air arising in the tank of liquid and being captured in pockets on the revolving rotary biological contactor. In Holmberg Pat. No. 3,956,127 and in the Carter Biodrum, drive is provided by a motor connected to a sprocket on the shaft of the rotary biological contactor, and adjacent a shaft positioning arm which is free to swing up and down as the rotary biological contactor floats freely in the liquid in the tank.
The extremely important service life of the rotary biological contactors is highly important because the rotary biological contactors are extremely expensive and must operate continuously, day and night, for many years, preferably without any service at all.
Problems have been encountered in the rotary biological contactor shafts which are mounted in bearings at their ends, wherein the shafts will fracture, as to render the whole rotary biological contactor useless. Such fracturing occurs because the shaft carries a substantial amount of weight, and extends the whole length of the tank. As a result, the shaft, mounted in end bearings, will continuously flex, during each revolution, and in time, the flexing causes the shaft to fail, by cracking and breaking. The answer to this in response to such shaft failure has been to put more steel into the shaft, thus attempting to make it stronger and less likely to fail. However, shaft failures continue to occur because the flexing of the shafts continues.
Hollow shafts such as seen in Thissen Pat. No. 4,115,268 provide some improvement because they are larger in diameter and tubular in construction, thus resisting flexing to a greater degree.
Such shaft flexing is accentuated under certain circumstances, due to non-uniform build-up or distribution of the heavy bio-mass on the surface of the plastic media. The bio-mass may be non-uniform as between one end of the media and the other end of the media, or non-uniform along the length of the plastic media. Similarly, the bio-mass may be non-uniform at one side of the plastic media as compared to the other side of the plastic media. Such side to side non-uniformity may be caused by unintended shut down of the processing unit, caused by accidental power failure, or similar causes. If the rotary biological contactor is stopped for a period of time, the bio-mass exposed to the air may dry out and flake off the plastic media, whereas the portion of the bio-mass immersed in the waste water tank will remain wet and heavy, with the result that when the rotary biological contactor is again started, one side will be substantially heavier than the other, causing significant asymmetric loading of the shaft.
In rotary biological contactors with stationary bearing-mounted shafts, such as in the Gass patent, the side to side non-uniform bio-mass tend to cause oscillatory vertical forces and unusual torques on the shaft which contribute materially to shaft and media deterioration and failure.
Failure of the shafts also occur in the free floating rotary biological contactors as illustrated in Holmberg Pat. No. 3,956,127, and commercially embodied in the Carter Biodrum, which has a single shaft extending through two adjacent buoyant rotary biological contactors (with plastic balls); and the shaft being driven with the chain and sprocket drive at a location between the two rotary biological contactors.
Failure of the shafts in the Holmberg rotary biological contactors and in the Carter Biodrum rotary biological contactor will occur due to such non-uniform build-up of the bio-mass on the rotary biological contactors so that one end carries a greater weight of bio-mass than the other end, so that one side of the rotary biological contactor has a substantially greater bulk of bio-mass than the other side.
In the free floating rotary biological contactors as in the Carter Biodrum and Holmberg Pat. No. 3,956,127, if one end of the rotor is more heavily laden with bio-mass than the other end, then one of the rotors will tend to float higher in the tank of waste water, and the other end will tend to be submerged further into the water. The center arm, between the two cages, which tends to hold the shaft provides restraint against one end raising and the other end lowering, with the result that the two ends of the shaft extending through the two rotary biological contactors, act as flexing cantilever arms, and they will flex during each revolution of the rotor, and eventually cause fracturing and failure over a period of time. This flexing of the shaft in the free floating rotary biological contactors is accentuated if the bio-mass becomes non-uniform in a direction transverse to the rotation axis, whereupon the rotary biologicial contactors will tend to oscillate upwardly and downwardly, or bounce, as they revolve in the tank of the waste water, and this bouncing, or vertical oscillation of the rotary biological contactors accentuates the flexing of the shaft, contributing in a greater degree to failure of the shaft.
The non-uniform build-up or distribution of the bio-mass on the plastic media also has a tendency to cause deterioration of the plastic media, in addition to the deterioration of the steel shafts. In such disc type plastic media as illustrated in Prosser Pat. No. 3,886,074 and Gass Pat. No. 3,827,559, if the bio-mass is non-uniformly distributed in a direction transversely of the rotation axis, the torque applied from the shaft onto the plastic media requires a greater strain to be applied to the plastic media in certain areas of the plastic media as compared to other areas. This non-uniform application of stress in the plastic media also causes flexing of the media and eventual deterioration and failure of it as by cracking and breaking.
The rotary biological contactors which are free floating, such as Holmberg Pat. No. 3,956,127 and the Carter Biodrum, will accommodate the overall build-up of greater quantities of bio-mass on the plastic media, and the rotary biological contactors are permitted to submerge to the extent that 60% of the rotary biological contactor may be immersed in the waste water. However, when this situation occurs, the central portion of the plastic media, that is, the plastic balls in the cage closest to the shaft, are never permitted to arise from the waste water in the tank and be exposed to air. The failure of a portion of the plastic balls or plastic media to be exposed to air defeats the purpose of the rotary biological contactor, and does not permit the rotary biological contactor to function as it is intended to function.
In summary, the rotary biological contactors which have been used by commercial and municipal was water treatment systems have experienced a substant frequency of shaft failures, both in shafts which are solidly mounted in bearings at their ends above the water level; and in shafts of rotary biological contactors which are free floating under the buoyant effect of the plastic media which permits the rotary biological contactors to vary the degree of submergence in the waste water.
Further, in summary, the existing rotary biological contactors used in many commercial and municipal waste water treating systems cause excessive deterioration of the plastic media, and permit central portions of the plastic media to remain submerged at all times if there is a significant build-up of bio-mass on the buoyant plastic media. Such buoyant plastic media as plastic balls drastically reduce the surface area upon which the bio-mass may build-up because all of the inside faces of the plastic balls remain concealed and isolated from the waste water.